1. Technical Field
This invention relates to the field of drivers for semiconductor chips. More specifically, the invention relates to automatic off-chip driver adjustment based on load characteristics.
2. Background Art
An off-chip driver is the very last circuit before a signal leaves a chip and this driver drives data onto a board, wire, to another chip, etc. If the driver is a very strong driver, it has low impedance and high current. If such a driver is hooked up to a light load (in terms of capacitance, for instance), the driver over-drives the load and there will be instability, which causes ringing and excessive noise. On the other hand, if the driver is not strong enough, the driver will under-drive the load, and the performance suffers. The best driver design matches the driver""s impedance to the load""s impedance, ideally at the point where response of the system is at critical damping. At this point, the off-chip driver has the most performance with no ringing or other detrimental effects. System designers actually tend to use a little under damping, as this allows for variance in load impedance.
There has been some movement in recent years towards programmable off-chip drivers. For instance, drivers exist that can be set to a specific impedance by electrical adjustment (such as by connecting wires to the driver or programming it). However, this technique requires human intervention.
What is needed is an automatic off-chip driver that requires little or no human intervention and that adjusts itself, based on load characteristics, to the optimum driver strength and impedance.
To overcome these problems, the current invention provides an automatic driver adjuster and methods using the same that modify off-chip drivers based on load characteristics. The preferred embodiments are preferably automatic and require little or no human intervention. Preferred embodiments of the current invention analyze and determine the impedance of a node and adjust a number of output drivers in response to the impedance of the node, or analyze a resultant waveform of the node, caused by an input waveform, and adjust a number of output drivers in response to the resultant waveform of the node.
In this manner, the proper number and strength of drivers may be automatically chosen for the current load characteristics of impedance or signal waveforms. Should the load characteristics change, the mechanisms of the current invention may be used to ensure that the driver has the optimal drive strength to meet these changing load characteristics.
The foregoing and other features and advantages of the present invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.